How to get the params from an http requests when unit testing in angularjs?

I have defined a method for my controller, like

$scope.submitForm = ( username, password ) =>{
    $http({ 
        method: 'post',
        url: 'balabala',
        params: {username, password}
    }).success( res => { /* */ } );
}

My test specs look like:

descript('myController', () => {
    beforeEach(module('myModule'));

    let controller, httpBackend, http, scope;

    beforeEach(inject(($controller, $httpBackend, $http, $scope) => {
        scope = $scope.$new();
        httpBackend = $httpBackend;
        http = $http;
        controller = $controller;

        httpBackend.when('POST', '/login')
            .respon({
                result: 'ok'
            });
    }));

    it('should POST login', () => {
        httpBackend.expectPOST('/login');
        const myController = controller('myController', {
            $scope: scope,
            $http: http
        });
        scope.submitForm();
        httpBackend.flush();
    });
});

How can I make sure that username and password have been posted?